Pronunciation: (nOO-tā'shun, nyOO-),[key] —n. 1. an act or instance of nodding one's head, esp. involuntarily or spasmodically.
2. Bot.spontaneous movements of plant parts during growth.
3. Astron.the periodic oscillation observed in the precession of the earth's axis and the precession of the equinoxes.
4. Mech.the variation of the inclination of the axis of a gyroscope to the vertical.